There were eyebrows raised when Southgate took charge of the national team in September 2016, but England have lost just twice since his arrival.
Mills has claimed that Southgate's ability to make "big decisions" has left the Three Lions in a strong position entering their 2018 World Cup opener against Tunisia on June 18.
"When Gareth came in people said he got the job because he was a yes man, that he wouldn't stand up and make ruthless decisions," Mills told Sky Sports News.
"I think we've seen that's not the case. He's left people out of his squad, that some people said should have been in it, [Jack] Wilshere, [Jonjo] Shelvey. He dropped Wayne Rooney very early on and has made big decisions.
"What he has done is creating an atmosphere, the players are relaxed. They're starting to enjoy playing for England again. Just look at the performance last night, it was energetic, they tried things."
England recorded a 2-0 win over Costa Rica on Thursday night in their final warm-up match ahead of this summer's tournament.